A variety of parallel optical communications modules exist for simultaneously transmitting and/or receiving multiple optical data signals over multiple respective optical data channels. Parallel optical transmitter modules have multiple optical transmit channels for transmitting multiple respective optical data signals simultaneously over multiple respective optical waveguides (e.g., optical fibers). Parallel optical receiver modules have multiple optical receive channels for receiving multiple respective optical data signals simultaneously over multiple respective optical waveguides. Parallel optical transceiver modules have multiple optical transmit and receive channels for transmitting and receiving multiple respective optical transmit and receive data signals simultaneously over multiple respective transmit and receive optical waveguides.
The transmit (Tx) portion of a parallel optical transmitter or transceiver module includes a laser driver circuit and an array of laser diodes. The laser driver circuit outputs an electrical drive signal to each respective laser diode to cause the respective laser diode to be modulated. When the laser diode is modulated, it outputs optical signals that have power levels corresponding to logic 1s and logic 0s. An optics system of the optical transceiver or transmitter module focuses the optical signals produced by each respective laser diode into the end of a respective transmit optical fiber held within an optical connector module that mates with a receptacle of the optical transceiver module.
The receive (Rx) portion of a parallel optical receiver or transceiver module includes at least an array of receive photodiodes that receive incoming optical signals output from the ends of respective receive optical fibers held in an optical connector module that mates with a receptacle of the optical receiver or transceiver module. The optics system of the transceiver or receiver module focuses the light that is output from the end of each receive optical fiber onto the respective receive photodiode. The respective receive photodiode converts the incoming optical signal into an electrical analog signal. An electrical detection circuit, such as a transimpedance amplifier (TIA), receives the electrical signal produced by the receive photodiode and outputs a corresponding amplified electrical signal, which is processed by other circuitry of the RX portion to recover the data.
In most parallel optical communications modules, the receptacle with which the optical connector module mates constitutes an electromagnetic interference (EMI) open aperture that allows EMI to escape from the housing of the parallel optical communications module. The Federal Communications Commission (FCC) has set standards that limit the amount of electromagnetic radiation that may emanate from unintended sources. For this reason, a variety of techniques and designs are used to shield EMI open apertures in module housings in order to limit the amount of EMI that passes through the apertures. Various metal shielding designs and resins that contain metallic material have been used to cover areas from which EMI may escape from the housings. So far, such techniques and designs have had only limited success, especially with respect to parallel optical communications modules that transmit and/or receive data at very high data rates (e.g., 10 gigabits per second (Gbps)).
For example, EMI collars are often used with pluggable optical communications modules to provide EMI shielding. The EMI collars in use today vary in construction, but generally include a band portion that is secured about the exterior of the transceiver module housing and spring fingers having proximal ends that attach to the band portion and distal ends that extend away from the proximal ends. The spring fingers are periodically spaced about the collar. The spring fingers have folds in them near their distal ends that cause the distal ends to be directed inwards toward the transceiver module housing and come into contact with the housing at periodically-spaced points on the housing. At the locations where the folds occur near the distal ends of the spring fingers, the outer surfaces of the spring fingers are in contact with the inner surface of the cage at periodically spaced contact points along the inner surface of the cage. Such EMI collar designs are based on Faraday cage principles.
The amount of EMI that passes through an EMI shielding device is proportional to the largest dimension of the largest EMI open aperture of the EMI shielding device. Therefore, EMI shielding devices such as EMI collars and other devices are designed to ensure that there is no open aperture that has a dimension that exceeds the maximum allowable EMI open aperture dimension associated with the frequency of interest. For example, in the known EMI collars of the type described above, the spacing between the locations at which the distal ends of the spring fingers come into contact with the inner surface of the cage should not exceed one quarter wavelength of the frequency of interest that is being attenuated. Even greater attenuation of the frequency of interest can be achieved by making the maximum EMI open aperture dimension significantly less than one quarter of a wavelength, such as, for example, one eighth or one tenth of a wavelength. However, the ability to decrease this spacing using currently available manufacturing techniques is limited. In addition, as the frequency of optical communications modules increases, this spacing needs to be made smaller in order to effectively shield EMI, which becomes increasingly difficult or impossible to achieve at very high frequencies.
In general, all of the current techniques of providing EMI shielding in optical communications modules attempt to ensure that there are no EMI open apertures that have dimensions that exceed the maximum allowable EMI open aperture dimension. As indicated above, as the frequencies or bit rates of optical communications modules continue to increase (i.e., wavelengths continue to decrease), it becomes extremely difficult or impossible to effectively implement these types of solutions. Accordingly, a need exists for an EMI shielding system and a method that do not rely solely on such techniques and that provide an effective EMI shielding solution in optical communications modules.